1. Field of the Invention
This invention relates to a catalyst for the aromatization of alkanes to aromatics, specifically a zeolite, preferably a MFI-type structure, most preferably a ZSM-5 MFI zeolite, catalyst for the aromatization of alkanes having two to six carbon atoms per molecule to aromatics, such as benzene, toluene and xylene.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may also contain other metals, such as sodium, calcium, barium, and potassium, and that has ion exchange properties (Encarta® World English Dictionary [North American Edition]© & (P) 2001 Microsoft Corporation). A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of said zeolite form. Much zeolite research has focused on the synthesis of zeolite frameworks containing elements other than silicon and aluminum.
U.S. Pat. No. 6,160,191 discloses that the term “zeolite” includes not only aluminosilicates but substances in which the aluminum is replaced by gallium, titanium, iron or boron and substances in which silicon is replaced by germanium, tin and phosphorous. U.S. Pat. Nos. 3,329,480 and 3,329,481, both issued to D. A. Young, report the existence of crystalline zirconosilicate and titanosilicate zeolites. A zeolite having chromium in the tetrahedral positions has been described by Yermolenko et al at the Second Oil Union Conference on Zeolites, Leningrad, 1964, pages 171–8 (published 1965). However, D. W. Breck, in Zeolite Molecular Sieves, p. 322, John Wiley & Sons (1974) suggests that the chromium present was not present in a zeolite A structure and furthermore was present as an impurity in insoluble form. The impurity was said to be in the form of a chromium silicate as confirmed by the nature of the water vapor adsorption isotherm.
The zeolite ZSM-5 has been synthesized with many elements other than Al in the framework, including iron. Synthesis of an iron-containing zeolitic structure was reported in Japanese Kokai 59,121,115, published Jul. 13, 1984, which disclosed an aluminosilicate having a faujasite structure and containing coordinated iron. The chemical composition is said to be of the formula aM2/nO:bFe2O3:Al2O3:cSiO2 where M can be H, alkali metal or alkaline earth metal; the symbol n is the valence of M; a=1+/−0.3; c is between 4.6 and 100; and a is less than b and both are less than 7. The crystal lattice parameter ao is between 24.3 and 24.7 angstroms. Similarly, U.S. Pat. No. 4,208,305 discloses crystalline silicates which are structurally a three-dimensional network of SiO42−, FeO42− and, optionally, AlO4−, GaO4− and GeO4− tetrahedrons, which are interlinked by common oxygen atoms and are of the overall composition in dehydrated form:(1.0+/−0.3)(R)2/nO[aFe2O3bAl2O3cGa2O3]y(d SiO2 eGeO2),where R is a cation; a≧0.1; b≧0; c≧0; a+b+c=1; y≧10; d≧0.1; e≧0; d+e=1; and n is the valence of R. Silicates not containing gallium, germanium and aluminum are preferred. Silicates of a particular X-ray powder diffraction pattern are also preferred.
U.S. Pat. No. 4,713,227 discloses crystalline metalloaluminophosphates having microposity, catalytic activity and ion-exchange properties which contain metals such as arsenic, bismuth, cobalt, iron, germanium, manganese, vanadium and antimony within the framework.
U.S. Pat. No. 4,704,494 discloses a process for conversion of low molecular paraffin hydrocarbons to aromatic hydrocarbons using a platinum- or gallium-modified metallosilicate (Si/Me) catalyst where Me is aluminum, gallium, titanium, zirconium, germanium, lanthanum, manganese, chromium, scandium, vanadium, iron, tungsten, molybdenum, nickel or a mixture thereof.
U.S. Pat. No. 5,456,822 discloses a process for aromatization of hydrocarbons containing two to nine carbon atoms per molecule with a catalyst containing an MFI zeolite having silicon, aluminum and/or gallium in the framework, a matrix, and gallium, a noble metal of the platinum family and a metal selected from tin, germanium, indium, copper, iron, molybdenum, gallium, thallium, gold, silver, ruthenium, chromium, tungsten and lead deposited on the zeolite.
U.S. Pat. No. 4,180,689 discloses a zeolite composition for producing aromatic hydrocarbons from aliphatic hydrocarbon feedstock. The zeolite contains gallium which has been exchanged for a cation or proton or impregnated into the zeolitic cavities. This patent teaches that improved yields of aromatic hydrocarbons are realized over zeolites which contain gallium in the form of its oxide substituted either partially or wholly for the aluminum oxide and being part of the crystal structure or the zeolite. There is no disclosure of deposit of platinum on the zeolite.
U.S. Pat. No. 4,851,602 discloses an oligomerization catalyst for conversion of alkanes and alkenes to high octane gasoline with medium pore (about 5–7 angstroms) shape selective crystalline aluminosilicate zeolites. Representative of the shape selective zeolites are ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, and ZSM-48. The zeolites may be ion exchanged or impregnated with one or more suitable metals, such as Ga, Pd, Zn, Ni, Co and/or other metals of Periodic Groups III to VIII and may include other components, generally one or more metals of group IV, IIB, IIIB, VA VIA or VIIIA of the Periodic Table (IUPAC) such as platinum and other noble metals, such as palladium, gold, silver, rhenium or rhodium. There is no disclosure of gallium as part of the zeolite crystal structure.
U.S. Pat. No. 5,149,679 discloses an intimate mechanical mixture (e.g., ballmilling) of a suitable zeolite (ZSM-5, ZSM-11 or ZSM-12) with a suitable gallium-containing compound to form a gallium containing zeolite catalyst with gallium loadings as low as 2 wt % useful in aromatization of light paraffins. Noble metals such as rhenium, rhodium, nickel, palladium, platinum and iridium may be present in the catalyst but are not necessary. There is no disclosure of gallium as part of the zeolite crystal structure.
U.S. Pat. No. 5,192,728 discloses tin containing microporous crystalline materials which have the same structure as zeolites, such as ZSM-5, and can contain other elements such as boron, iron, chromium and gallium (0–10 wt %). The catalyst may also include a hydrogenation/dehydrogenation metal such as platinum. A Pt/Ga-ZSM-5 catalyst disclosed in comparative data for hexane and heptane aromatization was reported to produce aromatics in low yields and to form C3 and C4 products by cracking.
U.S. Pat. No. 5,574,199 discloses a process for shape selective aromatization of low molecular weight olefins and paraffins to para-xylene with catalytic molecular sieves, such as intermediate pore zeolites, for example, ZSM-5, ZSM-11, ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50, and ZSM-57 with ZSM-5 being preferred, which may be modified with a hydrogenation/dehydrogenation metal selected from Groups IB to VIII of the Periodic Table, including platinum, palladium, nickel, copper, cobalt, gallium, molybdenum, rhodium, ruthenium, silver, gold, mercury, osmium, iron, zinc, cadmium, and mixtures thereof with gallium, zinc and platinum being preferred and gallium being most preferred. The catalyst must have been at least twice contacted with a silicon-containing selectivating agent and calcined prior to the aromatization process.
U.S. Pat. No. 5,932,777 discloses a multi-step reaction/separation process for converting hydrocarbons to aromatics with a catalyst of a zeolite, such as ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-35, ZSM-38, and combinations thereof, which may contain a promoter such as boron, phosphorus, sulfur, gallium, indium, zinc, chromium, silicon, germanium, tin, lead, lanthanides (including lanthanum) or combinations of thereof, which is preferably impregnated on the zeolite. There is no disclosure of gallium as part of the zeolite crystal structure.
U.S. Pat. No. 5,693,215 discloses crystalline borosilicate molecular sieves, i.e., “low-aluminum boron beta zeolite, in which the boron in the crystalline network may be replaced, at least partially, by aluminum, gallium or iron. Hydrogenating components, such as tungsten, vanadium, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal, such as palladium or platinum, may be present in the catalyst which is useful in catalytic cracking, hydrocracking and olefin/aromatic formation reactions.
The article “NAS (novel aluminosilicates) as catalysts for the aromatisation of propane, Studies of zinc and gallium modified zeolite-based systems having various extents of XRD crystallinity”, C. P. Nicolaides, N. P. Sincadu, M. S. Scurrell, Catalysis Today, volume 71, pages 429–435 (2002), indicates that conversion and BTX selectivity of PtGa-ZSM-5 catalysts are lower than for platinum free Ga-ZSM-5 catalysts and that Ga-ZSM-5 have similar propane conversion but higher BTX selectivity than for H-ZSM-5 catalysts.
The article “Light alkanes aromatization to BTX over Zn-ZSM-5 catalysts, Enhancements in BTX selectivity by means of a second transition metal ion”, Louis M. Lubango, Mike S. Scurrell, Applied Catalysis A: General, volume 235 pages 265–272 (2002) refers to platinum-containing catalysts, such as PtGa-ZSM-5, as exhibiting hydrogenolysis resulting in the formation of lower alkanes, e.g., ethane and methane from propane, during aromatization.
It would be advantageous to have a zeolite-type catalyst which has good selectivity for conversion of lower alkanes to aromatics, such as benzene, toluene and xylene, and a high content of ethane in the fuel gas byproduct.